Creep of selenium near the glass temperature

Creep experiments were carried out on amorphous selenium (Se) at temperatur es in the vicinity of the glass temperature. The recoverable compliance lac ks a plateau, indicating Se chains are too short to form an entanglement ne twork. The measured compliance function was thermorheological complex, even after subtraction of the glassy level and normalizing by the steady state compliance. The temperature dependence determined from the viscosity was in accord with previous viscosity data, although weaker than the near-Arrheni us dependence deduced from the stress relaxation of Se. Based on a comparis on to other, small-molecule glass-formers, the dynamic fragility calculated from the viscosity was larger than expected from Se's thermodynamic fragil ity (i.e., steepness of the normalized Kauzmann curve). In contrast, althou gh polypropylene (PP) is substantially more dynamically fragile than Se, PP is less thermodynamic fragile. Thus, when compared to either small-molecul e liquids or polymers, Se exhibits a disconnect between dynamic and thermod ynamic measures of fragility. (C) 1999 American Institute of Physics. [S002 1-9606(99)50144-X].